You are trapped in a dungeon, and see two doors in front of you. One is a way out, andther will lead to certain death.
Two people are standing in front of these doors, one of them a Knight, another - a Liar. Naturally, you do not know who is who, but they know each other's true nature.
Both the people know which way is correct, but you can only ask one question of one of them before making your choice.
What will you ask?
Point to one of the doors and posit to each of the individuals the following query, " If I told you that this door is the way out, would you say yes?"
If the door id indeed the way out then each person would answer "yes", while if the door lead to one's doom, both the individuals would respond "no".